The present invention relates to a method of manufacturing a semiconductor element such as a thin film transistor used as a drive element for an image sensor or for a flat panel display, and a photo diode used as a photosensing element for an image sensor. More particularly, the present invention relates to a method of manufacturing a semiconductor element improved such that when the semiconductor element is a thin film transistor, its reliability is improved by minimizing a variation of the values of threshold voltage (Vth) as its key characteristic, and when the semiconductor element is a photo diode, its dark/bright current ratio (S/N ratio) is improved.
A conventional thin film transistor of the inverse stagger type uses an amorphous silicon hydride (a-Si:H) as a semiconductor active layer, as shown in a cross section of FIG. 1.
In the structure of this type of thin film transistor (TFT), a gate electrode 2 formed of chromium (Cr) or tantalum (Ta), a gate insulating layer 3 formed of a silicon nitride (SiNx) film, a semiconductor active layer 4 formed of nondoped amorphous silicon hydride (i-a-Si:H), a channel protecting layer 5 formed of a silicon nitride (SiNx) film, an ohmic contact layer 6 formed of n.sup.+ amorphous silicon hydride (n.sup.+ a-Si:H), a source electrode 7 and a drain electrode 8, an interlayer insulating layer 9 formed of polyimide, and a wiring layer 10 formed of molybdenum (Mo), aluminum (Al) or the like are layered on a glass substrate 1 in this order.
In the thin film transistor thus structured, a state of the interfaces between the gate insulating layer 3 and the semiconductor active layer 4 and between the semiconductor active layer 4 and the channel protecting layer 5 decisively determines the TFT characteristics. Accordingly, it is known that contaminants of the interfaces and electrostatic charges in the vicinity of the interfaces have a great influence on the threshold voltage Vth of the thin film transistor.
A measure practically employed in the manufacturing of the semiconductor element is to successively form those three layers, the gate insulating layer 3, the semiconductor active layer 4, and the channel protecting layer 5 in a vacuum condition. The measure succeeds in preventing the interfaces from being contaminated and hence in minimizing a variation of the TFT characteristics of the manufactured semiconductor element.
A conventional method of manufacturing a thin film transistor will be described with reference to FIGS. 2(a) to 2(d).
The gate electrode 2 of chromium (Crl) is formed on the glass substrate 1. On the structure, silicon nitride (SiNx) layer of the underlayer as the gate insulating layer 3, an amorphous silicon hydride (a-Si:H) layer as the semiconductor active layer 4, and a silicon nitride (SiNx) layer of the upper layer as the channel protecting layer 5 are successively formed in a vacuum state by a plasma CVD method. The SiNx layer of the upper layer is patterned to form the pattern of the channel protecting layer 5 (see FIG. 2(a)).
An n.sup.+ a-Si:H layer as the ohmic contact layer 6, a chromium (Cr2) layer to serve as the source electrode 7 and the drain electrode 8 are formed. The n.sup.+ a-Si:H layer, the Cr2 layer, and the a-Si:H layer as the semiconductor active layer 4 are successively patterned to form the semiconductor active layer 4, the ohmic contact layer 6, and the source electrode 7 and the drain electrode 8 (see FIG. 2(b)).
Then, polyimide (PI) is coated by a spin coating method to form a polyimide layer of approximately 1 .mu.m thick to be used as the interlayer insulating layer 9. The coated layer is prebaked for the initial hardening, and then patterned to form, for example, contact holes reaching the source and drain electrodes. Finally, it is post baked to complete the interlayer insulating layer 9 (FIG. 2(c)).
Aluminum (Al) or the like is vapor deposited and patterned to form the wiring layer 10 and the like (FIG. 2(d)). In this way, the conventional thin film transistor of the inverse stagger type is manufactured (reference is made to Japanese Patent Unexamined Publication No. Hei. 3-157970).
However, in the above mentioned TFT manufacturing method, when the interlayer insulating layer 9 is formed by using an organic insulating material such as polyimide (PI), molecules in the polyimide film, immediately after the polyimide coating process, are frequently polarized by an electrostatic field existing therearound. Particularly, when the PI molecules are baked in a state that those are oriented in the direction vertical to the glass substrate 1, the molecules remain not electrically neutralized and are left in the form of fixed charges (see Extended Abstract of the 22nd (1990 International) Conference on Solid State Devices and Materials, Sendai, 1990, pp 1039-1042).
The fixed charges resulting from the polarization of the polyimide molecules will be described with reference to FIGS. 3 to 5. FIG. 3 is a sectional view for explaining fixed charges oriented in a fixed direction, which result from the polarization of polyimide molecules, FIG. 4 is a sectional view showing a thin film transistor in which a top gate electrode 21 is provided above the channel protecting layer 5. FIG. 5 is a graph showing a variation of the threshold voltage Vth with respect to a voltage applied to the top gate electrode 21 of the thin film transistor of FIG. 4.
In the illustration of FIG. 3, by electrostatic charges or the like, the polyimide molecules of the interlayer insulating layer 9 are polarized such that positive charges stay in the upper region of the insulating layer and negative charges stay in the lower region thereof. The interlayer insulating layer 9 is baked in a state that the polarized polyimide molecules are oriented vertically with respect to the glass substrate 1. Due to the fixed charges thus formed, charges are induced in the interface between the channel protecting layer 5 and the semiconductor active layer 4. As a result, the threshold voltage Vth of the thin film transistor is shifted by a quantity necessary for canceling the fixed charges. In the case of FIG. 3, the threshold voltage Vth is increased as compared with a case where the interlayer insulating layer 9 does not include fixed charges.
This state of fixed charge generation in the thin film transistor is equivalent to the state of the thin film transistor when the top gate electrode 21 is provided above the channel protecting layer 5, and a voltage (top gate voltage) is applied to the top gate as shown in FIG. 4.
As shown in FIG. 4, the top gate electrode 21 is formed above the channel protecting layer 5, with the interlayer insulating layer 9 interlayered between them. In the thin film transistor thus structured, charges are induced in the interface between the channel protecting layer 5 and the semiconductor active layer 4 when a voltage is applied to the top gate electrode 21. As seen from the graph of FIG. 5, the threshold voltage Vth greatly varies in accordance with a variation of the voltage applied to the top gate electrode 21. Specifically, the threshold voltage Vth is inversely proportional to the gate voltage. From this fact, it is seen that a degree of the polarization of the polyimide molecules and the direction of the oriented polyimide molecules are important factors to determine the threshold voltage Vth.
Further, in the method of manufacturing the thin film transistor, the film forming temperature for the channel protecting layer 5 must be lower than the film forming temperature of the semiconductor active layer 4, in order to secure the protection of the semiconductor active layer 4. This fact hinders the film quality of the channel protecting layer 5. Accordingly, the interface between the semiconductor active layer 4 and the channel protecting layer 5 is more sensitive to the electrostatic charges or the like than the interface between the semiconductor active layer 4 and the gate insulating layer 3. Thus, the thin film transistor is greatly influenced particularly at the interface between the semiconductor active layer 4 and the channel protecting layer 5, by the fixed charges of the polarized polyimide.
Next, the structure of a photo diode used as a photosensing element of an image sensor will be described with reference to FIG. 6 showing a sectional view of the photo diode.
The photo diode, as shown in FIG. 6, is of the sandwich type in which a lower electrode 11 formed of a metal such as titanium (Ti) or chromium (Cr), a photoconductive layer 12 as an a-Si:H layer segmented for each photosensing element, and a transparent electrode 13 formed of an indium/tin oxide (ITO) layer segmented in a similar manner are successively layered on a glass substrate 1. An interlayer insulating layer 9 of polyimide is formed over the entire surface of the photosensing element. Further, a wiring layer 10 made of aluminum (Al) or the like is formed over the interlayer insulating layer 9.
A conventional method of manufacturing a photo diode will be described with reference to FIGS. 7(a) to 7(d) showing cross sections of the semiconductor structure in the manufacturing steps.
A chromium (Cr) layer with a thickness of about 1000 to 1500 Angstroms is formed on the glass substrate 1, and patterned to form the lower electrode 11 (FIG. 7(a)).
Then, an amorphous silicon hydride (a-Si:H) layer with a thickness of about 1 to 2 .mu.m is formed by a plasma CVD method. An ITO film with a thickness of about 800 Angstroms is deposited thereon by a DC sputtering method, and is patterned by photolithography and etching processes, thereby forming the transparent electrode 13. Using a regist pattern as a mask, the a-Si:H layer is etched to form the photoconductive layer 12 (FIG. 7(b)).
The surface of the semiconductor structure is coated with polyimide to form a polyimide film with a thickness of about 1 .mu.m, the polyimide film is prebaked, shaped to have a predetermined pattern, and postbaked, thereby forming the interlayer insulating layer 9 (FIG. 7(c)).
Subsequently, an aluminum (Al) film is formed on the interlayer insulating layer 9, and the Al layer is shaped by the photolithography and etching process to form the wiring layer 10 of a predetermined pattern (FIG. 7(d)). In this way, the conventional photo diode is manufactured.
The bright/dark current ratio of the photo diode thus manufactured will be described with reference to FIGS. 8 and 9. FIG. 8 is a cross sectional view showing the photo diode before the interlayer insulating layer 9 is formed. FIG. 9 is a graph showing the relationship of a bias voltage applied to the lower electrode 11 and a dark current.
As shown in FIG. 8, a bias voltage is applied between the lower electrode 11 and the transparent electrode 13 through the photoconductive layer 12 sandwiched by them. The positive polarity of the bias voltage is connected to the lower electrode 11, while the negative polarity is connected to the transparent electrode 13. As seen from FIG. 9, when the bias voltage thus applied to the lower electrode 11 increases, the dark current increases. When the bias voltage exceeds a certain voltage, the dark current sharply increases, viz., the bright/dark current ratio (S/N ratio) abruptly decreases.
The photo diode before the interlayer insulating layer 9 is formed, is described in FIGS. 8 and 9. The same thing as FIG. 9 is correspondingly applicable for the photo diode after the interlayer insulating layer 9 is formed, provided that the polyimide molecules of the interlayer insulating layer 9 are not polarized, viz., no fixed charges are generated. The reason for this is that the electric characteristics of the photo diode remain unchanged where no fixed charges are present. Accordingly, also in the photo diode where the polyimide molecules of the interlayer insulating layer 9 are not polarized, the bright/dark current ratio (S/N ratio) will abruptly decrease when the bias voltage exceeds a certain voltage, as shown in FIG. 9.
However, in the thin film transistors manufactured by the conventional manufacturing method as mentioned above, a degree of the polarization of the polyimide molecules and the orientation of the polarized molecules are not uniform owing to various factors in the manufacturing process. Accordingly, it is almost impossible to manufacture the thin film transistors of the constant value of the threshold voltage Vth (Vth) which is very sensitive to the fixed charges caused by the polarization of the polyimide molecules. That is, the manufactured thin film transistors have large variation in the threshold voltage Vth. Further, in an image sensor or a flat panel display device using the thin film transistors manufactured as described above, its output characteristics are degraded.
Further, in the photo diodes manufactured as described above, the dark current increases with an increase of the bias voltage applied between the transparent electrode and the lower electrode under the condition that the polyimide molecules of the interlayer insulating layer interlayered between those electrodes are not polarized. Particularly, when the bias voltage progressively increases, the dark current sharply increases, so that the S/N ratio is degraded. When an image sensor is constructed using those photo diodes as photosensing elements, the sensitivity of the image sensor is reduced.